Several television reception devices are currently on the market, notably satellite decoders. To receive programs, the receiver must identify the list of channels. According to a first method, the user manually configures its receiver by introducing the frequency ranges in operation at the place where the receiver is located. There are also automatic methods enabling a receiver to search for available channels. According to a simple mode, the automatic search consists of frequency scanning the entire reception band used by the device then memorizing all the carrier frequencies as well as some information relating to the received channel. For a satellite decoder, the reception band to be scanned may be for example 1 GHz and require a relatively long scanning.
To carry out the scanning of the band, it is known to scan per frequency step. The frequency step is fixed to be less than or equal to the width of a channel so that if a channel is present somewhere along the reception band it is obligatory to find it. During a frequency incrementation, the signal corresponding to the frequency is measured and compared to a threshold. If the signal is greater than the threshold, the frequency is varied positively and negatively in order to determine the carrier frequency of the channel. Then the channel is decoded in order to obtain and memorizes the information relating to the found channel.
The satellite transmissions use different channel widths. By way of example, the same satellite can broadcast channels of a width of 25.33 or 40 MHz. The scanning is thus carried out with a step corresponding to the shortest channel width. To identify the channel type, it is necessary to check during each frequency skip what channel has been found using a successive identification on each channel type.
The time needed for the operation is equal to N*Ts+M*Ti, with N being the skip number to be performed, Ts the time needed for the frequency change of the oscillator performing the band scan, M the number of channels present in the scanned band and Ti the average identification time of a channel. The scanning of the whole band can last several minutes.
The U.S. Pat. No. 6,362,772—SKOTCH DERWIN document describes a piece of equipment for receiving signals from a satellite. Satellite antennas are linked to a device scanning all the frequencies and transmitting the information to a server. The server is linked to the user devices. The associated antennas and filters are known to only receive transmissions from satellite sources, so that the problem of discrimination with the other sources does not exist.
The U.S. Pat. No. 6,732,061—WICHELMAN document describes the professional network heads. The network head carries out a spectral analysis by frequency step in order to check whether or not the transmission frequency has drifted. If the network head is no longer set on the right frequency, an automatic correction method is performed. This document does not enable the received channel type to be detected.
One purpose of the invention is to reduce the time needed for an automatic channel search on a device receiving channels of different widths. The invention proposes a channel detection method which comprises a fast measurement step of the whole frequency range, then measurement exploitation and filtration step for certain frequencies.